Magnetic high frequency attenuator



MarchS, 1957 E.F.HARR|S 2,784,382

MAGNETIC HIGH FREQUENCY ATTENUATOR Filed April 5, 1952 2 Sheets-Sheet 1A; "Lair-4" l. fw

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March 5, 1957 E. F. HARRIS 2,784,332

MAGNETIC HIGH FREQUENCY ATTENUATOR Filed April 5, 1952 2 Sheets-Sheet 2F -A q. 4 24' 62 MT Z5 Z4 a 4 L I 2 $7115 TJZLLIT Edward F ffizrn'sMAGNETIC HIGH FREQUENCY ATTENUATOR Edward F. Harris, Columbus, Ohio,assignor to Thompson Products, Inc., Cleveland, Ohio, a corporation ofOhio Application April 5, 1952, Serial No. 280,833

2 Claims. (Cl. 333-451) This invention relates to a magnetic highfrequency attenuator and more particularly to a structure forattenuating high frequency electro-magnetic energy in accordance withthe strength of a magnetic field.

According to this invention, an insulator matrix having finely dividedmagnetizable particles embedded therein is placed in a high frequencyelectro-magnetic energy transmission structure and subjected to avariable magnetic field. It has been found that such a matrix imposes ahigh degree of attenuation on the high frequency energy when themagnetic field is small or non-existent but the degree of attenuationdecreases rapidly as the magnetic field increases and, with a magneticfield of sufl'lcient strength, the degree of attenuation of the highfrequency energy will be extremely small. This structure is highlyadvantageous as a switching means and also as a means for modulating thehigh frequency energy in accordance with a control signal. Inparticular, no moving parts are used and the structure is, in practice,readily and economically manufacturable. A further advantage is thatthis structure can operate efiiciently over a wide range of frequencies.

In a preferred embodiment of this invention, the matrix is of annularform and is disposed around the inner conductor of a co-axialtransmission line and may provide a support means therefor. Oppositepoles of an electromagnet are disposed on opposite sides of the line toinduce a transverse field through the matrix.

As an alternative, a coil may be disposed around the matrix to providean axial field. Difi'iculty may be experienced in obtaining a field ofsufficient strength with this method, but it is entirely satisfactory(and desirable due to the simplicity with which it can be made andassembled) in instances where a strong field is not necessary ordesirable.

In order to reduce any eddy current losses, a section of the outerconductor surrounding the matrix is preferably removed and replaced byan annular member of dielectric material together with a shorting wireand capacitor combination. By proper selection of components, the inputand output impedance of the attenuator section may be made substantiallyconstant over a wide range of frequencies.

The magnetic field may be changed between predetermined constant valuesto provide a switch or may be continuously variable. If desired, thefield may be controlled by a low frequency signal source and a modulatorarrangement. ,In such a modulator arrangement, an A.-C. current aloneshould not be applied to the coil of the electro-magnet since theattenuation will decrease with increased magnetic field in eitherdirection and a frequency twice the frequency of the A.-C. current willappear in the output. This effect is eliminated, by this invention, byproviding a constant biasing voltage in series with the low-frequencysignal voltage.

An object of this invention, accordingly, is to provide improvedstructure to effect variable attenuation of high frequencyelectro-magnetic energy.

ited States Patent cording. to this invention.

Another object of this invention is to provide a variable coaxial lineattenuator operable over a wide range of input frequencies.

A further object of this invention is to provide high frequencyattenuator structure controlled by a low frequency control signal with asubstantially linear relation between the magnitude of the highfrequency energy output and the magnitude of the control voltage.

This invention contemplates other objects, features and advantages whichwill become more fully apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings which illustratepreferred embodiments and in which:

Figure l is an elevational view of a preferred type of attenuatorstructure used in conjunction with a coaxial line;

Figure 2 is a cross-sectional view taken substantially along lines II-IIof Figure 1;

Figure 3 is a sectional View through another preferred form ofattenuator;

Figure 4 is a schematic diagram of a circuit equivalent to the preferredforms of attenuators of Figures 1, 2 and 3;

Figure 5 is a schematic diagram of a preferred arrangement forcontrolling attenuation in accordance with a control signal; and

Figure 6 is a schematic diagram of another preferred arrangement ofusing the attenuators of this invention.

In Figures 1 and 2, reference numeral 10 generally designates apreferred type of variable attenuator ac- In the attenuator 10, aninsulator matrix 11 of annular form and having finely dividedmagnetizable particles embedded therein is disposed around the centralconductor 12 of a co-axial line section and an electro-magnet 14. havingan energizing coil 15 has opposite poles 16 and 17 disposed on oppositesides of the matrix 11. It has been found that, with no magnetizing fluxthrough the matrix 11, high frequency electro-magnetic energytransmitted into the coaxial line section will be highly attenuated bythe matrix 11. When the electro-rnagnet 14 is energized by cur.-

rent flow through the energizing coil 15, a transverse magnetic fluxwill be set up in the matrix 11 and the attenuation of high frequencyenergy by the matrix 11 will be greatly reduced. With sufiicient fluxdensity in the matrix 11, the attenuation of high frequency energy bythe matrix may be reduced to a negligible quantity.

To insure maximum flux in the matrix 11, and to reduce any eddy currentlosses, a portion of the outer conductor of the co-axial line section isremoved to define a pair of axially separated outer conductors 18 and 19with an annular member 20 of insulating dielectric material disposedbetween the conductors 18 and 19, around the matrix 11 and between thepoles 16 and 17 of the electro-magnet 14. It may be here noted that thematrix 11, in addition to serving its attenuating function, serves tosupport the outer conductors 18 and 19 and the member 20 in spacedrelation to the inner conductor 12. If desired, additional spacers 21and 22 of insulating material may be provided between the innerconductor 12 and the outer conductors 18 and 19, respectively.

To provide a current flow path between the outer conductors 18 and 19and to improve operation over a wide range of high frequency operatingfrequencies, the outer conductors 18 and 19 are preferably connected atone side by a shorting wire 23 and on the opposite side by a capacitor24 having leads 25 and 26 soldered or otherwise electrically connectedto the outer conductors 18 and 19, respectively.

Figure 4 is a schematic illustration of a circuit equivalent to theattenuator 10 in which lines 18 and 19 represent the outer conductors 18and 19, respectively. Line 12' represents the inner conductor 12,resistances 11a, 11b and lie represent the matrix 11, capacitances 11dand 11:: represent the capacitance of the matrix 11, inductances 25 and26' represent the leads 25 and 26, respectively, inductance 23'represents the inductance of the shorting wire 23, capacitance 20'represents the capacitance of the member 20, and capacitance 24represents capacitor 24.

It will be noted that the attenuator is thus equivalent to a resistivepi attenuator section in parallel with a reactive pi filter section.

With variations in the magnetic flux in the matrix 11, the resistancethereof as represented by the resistances 11a, 11b and 110 in Figure 4will vary in such a manner as to maintain a substantially constantcharacteristic impedance of the section over a wide range of attenuationratios and over a wide range of high frequencies. In practice, thecapacitance of the member 20 as represented by the capacitance 20',Figure 41, is negligible and the equivalent reactive pi filter sectionof the attenuator 10 is substantially a low pass filter with a seriesresonant circuit 25', 24 and 26' across the inductive arm 23, Figure 4.This filter section will have a substantially constant characteristicimpedance in the pass band and the series resonant circuit 25, 24- and26 preferably has a resonant frequency at or about the high frequencyend of the pass band so as to extend the high frequency range ofoperation with substantially constant characteristic impedance.

In Figure 3, reference numeral 30 generally designates another preferredform of attenuator according to this invention, reference numerals 11,12, 18, 1?, 20, 21, 22, 23, 24, 25 and 26 in this figure designatingparts identical to those described above in connection with Figures 1and 2. In this form, however, the electro-magnet 14 is replaced by acoil 31 disposed around the member 20 which may be excited to produceaxially extending lines of magnetic flux in the matrix 11. It has beenfound that the attenuator will operate in the same manner whether thelines of flux are transverse or axial. The arrangement of Figure 3,however, is not capable of inducing as high a flux, or capable ofreducing the attenuation to as high an extent, as the arrangement ofFigures 1 and 2. It is highly satisfactory and desirable, one to thesimplicity with which it can be made and assembled, where a high flux orhigh reduction in attenuation is not necessary or desirable.

In either of the preferred embodiments of attenuators described above,the magnetic field may be changed between predetermined constant valuesso that the attenuator acts as a switch. In the alternative, themagnetic field may be continuously variable. If desired, the field maybe controlled by a low frequency signal source in a modulatorarrangement. Such a modulator arrangement is highly advantageous incertain applications such as, for example, Where the high frequencyenergy is produced by a magnetron in which case the output amplitudecannot be varied between wide limits by any known arrangements.

Since the attenuation of the high frequency energy by the matrix 11 willdecrease with increased magnetic field in either direction, if an A.-C.current alone is applied to the coil in Figure l or the coil 31 inFigure 3, an amplitude modulation of the high frequency energy at twicethe frequency of the A.-C. current will result. This elfect may begreatly reduced, by this invention, by a circuit such as shown in Figure5 in which the control voltage is applied to terminals 33 and 34 withthe electromagnet coil 15 and a battery 35, or other source of constantvoltage, connected in series between the terminals 33 and 34. Theconstant voltage thus provides a bias such that variations in thevoltage applied to the terminals 33 and 34 of either positive ornegative potential will produce corresponding variations in theintensity 4 of the magnetic flux and the attenuation of the highfrequency energy.

Another manner of using; the attenuators of this invention isillustrated in Figure 6 in which a pair of the attenuators 10 aredisposed in co-axial lines 36 and 37 connected in parallel to an inputco-axial line 38. One

terminal of the coil of each attenuator 10 is connected to one terminalof a battery 39. The other terminal of the battery 39 is connected tothe movable contact 40 of a single pole-double throw switch 41 and theother terminals of the coils of the attenuators 10 are connected tostationary contacts 42 and 43 of the single poledouble throw switch 41.By actuation of the switch 41, accordingly, the coils of the attenuators10 are selectively energized to selectively reduce attenuation in thelines 36 and 37. This arrangement thus provides a switching structurefor co-axial lines with no moving parts in the high frequency circuit.

The matrix 11 may be of any desired insulating material withmagnetizable particles dispersed therein and may preferably be a ceramicmatrix of fused earth-like materials such as the silica, kaolin,feldspar and the like ingredients of porcelain containing finely dividedparticulate iron adapted to be magnetized while embedded in such matrix.The amount of iron may range from the minimum amount (i. e. about 0.5weight percent of the ceramic matrix) capable of imparting anappreciable attenuating effect to the maximum practical amount (i. e.about 50 weight percent of the matrix) which might be used withoutdestroying the insulating properties of the matrix, and preferably about1-5% is used.

Table I below sets forth the preferred ranges of magnetic properties forcompounds preferred for use in the invention and those properties ofcompound A (available commercially as Ferramic-B) which has been foundto give optimum results in the practice of the invention.

Table I Property Range Compound Initial permeability at 1 melsecond 70to 125 Maximum permeability 150-230 183 Saturation flux density (Gauss)1, 500-2, 500 1, 000 Residual magnetism (Gauss 6504, 830 Coercive ioree(Oerstnd) 2-4 Volume resistivity (ohm-cm.) 1. 53X10 2X10 Curie point(G.+) 200-300 260 Loss factor at 1 Ind/second 0. 0001-0. 0002 0. 00016Loss factor at 5 me./seeond 0. 0005-0. 0015 0.0011

The superior results obtained by the instant invention may bedemonstrated by providing a co-axial line having an inner conductor ofinch diameter and an outer conductor of approximately one half inchdiameter, placing a matrix of compound A with a diameter of one-halfinch and a length of one-half inch in the line, removing a pontion ofthe outer conductor around the matrix and replacing the same with anannular dielectric member, a shorting wire and a capacitance of 0.006mfd. in the manner as illustrated above in Figures 1 and 2. With nomagnetization of the matrix, the loss in the line due to powerabsorption by the matrix has been found to be approximately 20 db,while, with the matrix magnetized by a magnet as illustrated in Figures1 and 2 and a magnetic field of approximately 10,000 Gausses, theattenuation loss is reduced down to approximately 1 db. This arrangementis effective over a range of high frequencies from 100 to 1,000megacycles.

It will be understood that the compositions, dimensions and structuralarrangements as described above, although peculiarly and highlyadvantageous, are given by way of illustrative example only and thatmodifications and variations may be effected without departing from thespirit and scope of the novel concepts of the present invention.

I claim as my invention:

1. A variable attenuator for high-frequency electromagnetic energy,comprising: an inner conductor, a pair of aligned axially spaced outerconductors in radially spaced relation to said inner conductor andinsulated therefrom, an annular member of dielectric material alignedbetween said outer conductors, a matrix of annular form within saidannular member and around said inner conductor, a shorting wire at oneside of said annular member and connected at opposite ends to said outerconductors, a capacitor at the opposite side of said annular member andconnected between said outer conductors, said shorting wire, matrix andcapacitor cooperating to define a low pass filter section with asubstantially con stant characteristic impedance over a wide range ofoperating frequencies, said matrix being of insulating material andhaving magnetizable particles dispersed therein, and means external tosaid annular member for inducing a magnetic flux therethrongh and insaid matrix for controlling attenuation of high-frequency energy.

2. A co-axial line attenuator comprising: an inner conductor, a pair ofaligned axially spaced outer conductors in radially spaced relation tosaid inner conductor and insulated therefrom, a matrix of annular formaround said inner conductors and having opposite end portions disposedwithin said outer conductors for supporting said outer conductors inaxially spaced relation on said inner conductor, said matrix being ofmagnetic material arranged to attenuate high frequency energy to anextent dependent upon uni-directional flux there-through, and

an electro-magnet having opposite poles on opposite sides of said matrixand aligned between the facing ends of said axially spaced outerconductors for inducing a transverse flux in said matrix and controllingattenuation of high frequency energy, and a direct current conductiveconnection between said outer conductors at only one side of the fieldpath between said poles.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES NBS Magnetic Attenuator, Nat. Bureau of Standards TechnicalNews Bulletin, vol. 35, No. 8, pages 109411, August 1951.

